In recent years, research has been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of these light-emitting elements, a light-emitting substance is sandwiched between a pair of electrodes. By applying voltage to such a light-emitting element, light emission can be obtained from the light-emitting substance.
Since such a light-emitting element is a self-luminous type, there are advantages such as higher visibility of a pixel and no necessity of a backlight, compared to a liquid crystal display. Accordingly, such a light-emitting element is suitable for a flat panel display element. Another major advantage of such a light-emitting element is that it can be manufactured to be thin and lightweight. In addition, high response speed is also one of the features.
Furthermore, since such a light-emitting element can be formed into a film shape, plane light emission can be easily obtained. Therefore, a large-area element can be formed by utilizing the plane light emission. This is a feature that is hard to be obtained in point light sources typified by a filament lamp and an LED or linear light sources typified by a fluorescent light tube. Therefore, such a light-emitting element also has a high utility value as a plane light source that can be applied to a lighting device or the like.
Deterioration of a light-emitting element is one of the reasons why the above-described light-emitting element with many advantages and a light-emitting device including such a light-emitting element are limited to a partial practical use. A light-emitting element deteriorates such that the luminance is lowered in accordance with the accumulation of driving time even if the same amount of current is fed thereto. It is essential to obtain a light-emitting element whose degree of deterioration is acceptable for an actual product so that the light-emitting device can be widespread. A light-emitting element has been researched from many aspects such as aspects of a driver circuit, sealing, an element structure, and a material (for example, see Patent Document 1).